1. Field of the Invention
The present invention relates to a mullite ceramic composition, and more particularly, to a mullite ceramic composition suitable for use as a material for electronic parts, such as a circuit board or a ceramic package for a semiconductor device.
2. Description of the Related Art
Current requirements for a high signal propagation speed in electronic parts, high-density packaging, and low cost, mean that a package and a substrate material which form the electronic parts must have a low dielectric constant, a thermal expansion coefficient close to that of a semiconductor element, a high mechanical strength, a high thermal conductivity, and a low production cost.
To satisfy these requirements ceramics such as an aluminum nitride ceramic, a silicon carbide ceramic, a low-temperature-fireable ceramic and a mullite ceramic have been developed.
The mullite ceramic is fired (or sintered) by adding in solid solution an alkaline earth element compound such as magnesium oxide to mullite powder as a sintering aid, but in a flattering process using a reburning step to straighten a bent fired ceramic, a problems arises in that flecks or spots are generated on the surface of or within the ceramic.
Particularly, in a white mullite ceramic, the generation of flecks greatly deteriorates the value thereof as a commercial product, and even if a coloring agent is added to the mullite ceramic, inhomogeneity of color occurs due to the generated flecks.
The present inventors have developed a mullite ceramic composition using a rare earth element compound such as yttrium oxide as a substitute for the alkaline earth element compound, as a sintering aid, to improve the mechanical strength of the above-mentioned conventional mullite ceramic, as disclosed in JP-A-62-140444.
Nevertheless, although this mullite ceramic has a high mechanical strength (flexural strength) flecks are still generated on the surface of or within the ceramic.
Further, to obtain a strong brazing adhesion to a metallized layer formed on a mullite ceramic surface, a large amount of the rare earth element compound is required, but this inclusion of a large amount of the rare earth in the compound causes flecks to be generated in a firing step before the flattering process. Further, even if the amount of rare earth contained is reduced, although flecks are not generated in the first firing step, such flecks are generated in the flattering step using a second firing process.
Further, flecks are easily generated when the firing temperature is relatively low, and therefore, it is difficult to set the most suitable firing conditions.
Furthermore, even if a coloring agent is added to the ceramic, the same problem arises, i.e., inhomogeneity of color caused by the flecks.